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Home Battery Q&A

Answering the most common builder questions

Home Battery Q&A

You've decided you want battery storage. How do you go about choosing the right one for your customers? Battery makers will help you through the process, but you're more likely to make the right choice if you go into those discussions with some basic knowledge.

According to Adam Weinstein, a regional sales manager with battery-maker Sonnen, builders tend to ask a lot of the same questions when deciding what battery type and system to choose for their homes. We asked him what those questions are and how he generally answers them.

 

Q: What main types of home batteries are on the market today?

A: The two most common types for home storage are Nickel Magnesium Cobalt (NMC) and Lithium Iron Phosphate (LFP).

NMC batteries are made for rapid charge and discharge, and also pack a lot of power into a small, mobile package. Those qualities have made them popular with some electric vehicle manufacturers. However, news stories about thermal runaway, or explosions, in electric vehicles have made some homeowners reluctant to install them.

LFP batteries weigh more and cost more, so they're unsuitable for electric vehicles. However, this type of battery has a longer lifespan and is designed specifically for stationary applications with no chance of thermal runaway, making the chemistry inherently safer and ideal for home installation.

 

Q: How long do home batteries last?

A: Manufacturers' warranties are a good way to compare the service life of two batteries but the actual lifespan will depend on battery chemistry and how often it gets cycled (from completely charged to completely discharged and back). Manufacturers of LFP batteries claim their batteries to last for 10,000 to 15,000 cycles, or over 30 years on a home that charges and discharges it every day. An NMC battery, by contrast, might be limited to a few thousand cycles, or 5-10 years on the same home.

 

Q: How do I size the battery?

A: It depends on a few key factors: the size of the solar array being integrated into the home, the size of the loads that the homeowners intend to back up with the battery in the event of an outage and the number of daily loads needing power.

Here are three sample battery sizes along with the loads they will power in backup mode for a typical home.

Battery size: 4kW/6kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • Three extra outlets

 

Battery size:7kW/10kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • One television
  • Three extra outlets
  • One single-room air conditioning unit

 

Battery size:8kW/16kWh

  • Refrigerator
  • Microwave
  • Three lights
  • One computer
  • One tablet or phone
  • One television
  • Three extra outlets
  • One single-room air conditioning unit
  • Two fans
  • One Washer/Dryer combination

 

Q: Where can my battery be installed?

A: The biggest consideration is temperature and moisture. Most batteries will deliver full power in a temperature range of 40F to 120F, so if you build in an area that gets hot in summer or cold in winter you don't want to put them in an unheated room or in direct sunlight. Most people install them in a semi-conditioned basement or garage.

Batteries are available with outdoor-rated enclosures that protect them from the elements, but an outdoor rating is not a failsafe option.

Some batteries, specifically LFP, are safe enough to install in a conditioned living space with the wiring totally hidden. In fact, many of the LFP batteries Sonnen has installed in German homes are in living spaces as they're considered a status symbol.

 

Q: Do I have to wire the house differently?

A: If you intend to use the battery for backup power, the only difference is that the electrician will need to wire a sub-panel, called a protected loads panel, with the circuits you want to power in the event of an outage.

 

Q: What is the difference between an AC-coupled and a DC-coupled system?

A:  An AC-Coupled system has two inverters: one for the solar panels and one for the battery. All power flowing to and from the battery—whether from the grid or the solar panels—flows through the battery inverter.

AC-Coupled is the easiest way to add a battery to an existing PV system and also provides redundancy—if one of the inverters fails, you don’t lose both your solar and storage. Also, the AC-coupling allows homeowners to charge their batteries from multiple power sources: solar, the grid and even gas generators.

In a DC-coupled system, the battery is charged directly from the panels. The panels and the battery also share a single "hybrid" inverter. Because this system type requires one inverter, equipment costs are lower, making it popular for new construction specifically with new solar installations.

With a DC-Coupled system, the batteries will only be charged by the solar array.

 

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